Amorphous semiconductor materials, in particular silicon and germanium, have long been considered for use in photovoltaic devices, particularly in cells intended for converting solar energy into electricity. This is because of the lesser cost of these materials versus the very high cost of normally used highly purified crystal materials such as polycrystalline silicon. Crystalline silicon will hereinafter be called c-Si, as is usual in the field. Amorphous silicon cells, assembled by relatively inexpensive vapor deposition techniques, have been characterized by efficiencies of about 4-8%, whereas cells of c-Si have yielded efficiencies of up to about 18%, albeit at great cost.
Amorphous silicon, hereinafter called a-Si, is characterized by very short diffusion lengths for minority carriers, typically 0.1 .mu.m. The consequence of this is that, conventional a-Si p.sup.+ i n.sup.+ cells cannot be made thick enough to absorb photons efficiently, although a-Si has a larger absorption coefficient than does c-Si, without sharply reducing the minority carrier collection efficiency. This occurs because such thicknesses substantially exceed the depletion length of the charge fields adjacent the p.sup.+ i and n.sup.+ i junctions of the cell. These charge fields are critical to efficient carrier collection, especially in a-Si cells.
Amorphous silicon, as the term is used herein, refers to the hydrogen-containing amorphous silicon material formed by glow discharge in silane as described by Carlson in U.S. Pat. No. 4,064,521. Optionally, the silicon may contain other elements such as fluorine, as taught by Ovshinsky et al. in U.S. Pat. No. 4,226,898. a-Ge is intended to mean analogous mixtures of germanium.
The problem of inefficient collection of minority carriers is recognized in the field and various solutions have been proposed. For example, Kressel et al. in U.S. Pat. No. 4,070,206 taught an a-Si tandem cell having an elongated optical path. The cell of Kressel et al has a stack comprising first and second layers and therebetween a third layer, the third layer being of opposite conductivity type to that of the others and having a thickness of at least twice the minority carrier diffusion length. Thus the cell has the p.sup.+ n p.sup.+ or n.sup.+ p n.sup.+ configuration and the junctions are in optical series. Electrodes collect minority carriers in the region of the junctions and are connected in parallel.
The Kressel et al cell has not been widely adopted because of the need to place the electrodes very close together (see IEEE Trans. Electronic Devices, ED-27 no. 4 April 1980) because of the very high sheet resistance characteristic of a-Si and because the p.sup.+ n junction in a-Si intrinsically has poor collection efficiency.
Chenevas-Paule et al addressed the same problem in U.S. Pat. No. 4,244,750 when they proposed a multilayer photovoltaic stack also characterized by a-Si junctions, in optical series, and transparent Schottky and ohmic contacts in parallel. Thus the proposed cell comprises a symmetrical arrangement of two basic photovoltaic stacks in optical series. Illustrative of the invention is a first basic stack comprising a semiconducting layer interposed between a supported lower layer forming a Schottky contact with said semiconductor layer, and a thin metal upper layer forming an ohmic contact with the same semiconductor layer. Sharing the same ohmic contact, a second basic stack of similar structure is placed on the first basic stack in reverse order to form complete cells. The two basic stacks may have different bandgaps whereby to capture a wider photon spectrum. The ohmic and Schottky contacts are connected in parallel and are semitransparent.
The Chenevas-Paule et al device would not function as well as would be desirable because minority carrier collection is not expected to be efficient, although it takes place in part within the depletion length of a charge field. This comes about because only one charge field, i.e., that field in the region of the Schottky-i junction, provides charge field assistance to minority carrier collection. Conventional two-junction cells of the p.sup.+ i n.sup.+ type having, of course, two charge fields generally collect minority carriers more efficiently.
In providing the thin metal layer ohmic contact joining the cells, the designer has the problem of choosing good transparency and low sheet resistance, and at the same time, providing good ohmic or Schottky contact depending on the embodiment.